This invention relates to optical communications systems, and more particularly, to a fiber-based apparatus and method for producing and/or detecting quantum correlated pairs of photons that can be manipulated to obtain various forms of entanglement, such as polarization entangled photons.
Efficient generation and transmission of quantum-entangled photon pairs, especially in the 1550 nm fiber-optic communication band, is of paramount importance for practical realization of quantum communication and cryptography protocols. The workhorse source employed in all implementations thus far has been based on the process of spontaneous parametric down-conversion in second-order (χ(2)) nonlinear crystals. Such a source, however, is not compatible with optical fibers as large coupling losses occur when the pairs are launched into the fiber. This severely degrades the entangled photon-pair rate coupled into the fiber, since the rate depends quadratically on the coupling efficiency. From a practical standpoint it would be advantageous if a photon-pair source could be developed that not only produces photons in the communication band but also can be efficiently spliced to standard telecommunication fibers. Over the past few years, various attempts have been made to develop more efficient photon-pair sources, but all have relied on the aforementioned χ(2) down-conversion process. Of particular note, one such prior art attempt used the effective χ(2) of periodically-poled silica fibers.
Counting single photons at the fiber telecom wavelengths of 1.3 μm and 1.5 μm poses significant problems for research, communications, and metrology applications. Single photon detection has been done either with photomultiplier tubes (which have low repetition rate and very low quantum efficiency) or with avalanche photodiodes. Avalanche photodiodes temporarily bias the diode above the breakdown voltage of the diode, and an avalanche then occurs either due to thermally generated carriers (a “dark count”) or from light generated carriers (a “light count”). The detector can be biased above breakdown until an avalanche occurs, then is reset. This method can either use active or passive quenching of the photodiode avalanche. There is a necessary inactive time after avalanche quenching needed for the electrons and holes to clear the diode. If the diode is reactivated before the avalanche clears, then the photodiode will breakdown from the carriers present from the previous avalanche. This afterpulsing limits the maximum rate that the diodes can be used at. Diodes can also be turned on at definite intervals. This is called gated-mode operation.
Detection of such short avalanches provides problems that were overcome in the 0.6 and 10 MHz systems. Due to capacitive coupling through the diode, there is a deterministic “ringing” output waveform that lasts for about the same time as the gate pulse. It is about 10-100 times larger than the avalanche. One known prior art system uses a pulse cancellation technique using reflected gate pulses. Another prior art system uses two diodes to obtain cancellation.
In light of the foregoing, it is an object of the present invention to provide a quantum/non-classical photon-pair source and/or method for such generation and transmission, overcoming various deficiencies and shortcomings of the prior art, including those outlined above.
Another object of the present invention is to provide a photon-pair source enhancing coupling efficiency and reducing coupling loss.
A further object of the present invention is to provide a photon-pair source and/or method of generation useful with standard telecommunication fibers.
A related object of the present invention is to provide a method of generating photon-pairs in useful communication bands, such bands as are compatible with a range of standard telecommunication fibers.
A further object of the present invention is to provide a method and/or apparatus useful therewith for the production of fluorescence photon pairs in a useful communication band.
Another object of the present invention, in accordance with one or more of the preceding objectives, is to provide an apparatus and/or source for the generation and/or transmission of photon pairs, such photon pairs as can be generated through use and employment of a suitable laser or incoherent pump of the sort or comparable to that described more fully below.
Yet another object of the present invention is to provide an apparatus, component and/or method of use for photon pair detection, such apparatus, component or associated method as can be utilized in accordance with various other aspects of this invention, including but not limited to a pulsed nature of the subject photon pairs.
Another object of the present invention is to provide a photon-pair source and/or method of generation useful for free space optics communication.
Another object of the present invention is to provide a photon-pair source and/or method of generation at any wavelength window from 0.4 microns to 2.0 microns through the use of a straight fiber or fiber Sagnac loop with dispersion zero in that window.
Another object of the present invention is to provide a source of multiple (at different wavelengths) photon-pair simultaneously and/or method of generation at any wavelength window from 0.4 microns to 2.0 microns through the use of a straight fiber or fiber Sagnac loop with dispersion zero in that window.
It is a further object of the present invention, alone or in conjunction with one or more of the preceding objectives, irrespective of apparatus or component used therewith, to provide a broad methodology for the generation of quantum-correlated photon pairs via fluorescent phenomena in the vicinity of the zero dispersion wavelength of fibers, such as the commercially-available dispersion-shifted fibers.
Other objects, features, benefits and advantages of the present invention will be apparent from this summary and the following descriptions of various preferred embodiments, and will be readily apparent to those skilled in the art having knowledge of various photon pair generation, quantum communication and/or encryption techniques. Such objects, features, benefits and advantages will be apparent from the above as taken in conjunction with the accompanying examples, data, figures and all reasonable inferences to be drawn therefrom.